Sensory information from the limb provided by mechanoreceptors in muscles, joints, and the skin is used for a broad range of sensory and motor functions. In particular, this sensory information combined with other sensory modalities, such as vision, and internal feedback of motor commands provides perceptual information relating to the body and limbs (Haggard and Wolpert, 2005), including position sense and kinesthesia. These perceptual features are sometimes called body scheme or body image.
Sensory feedback from the limb is also important for correcting errors in motor performance, referred to as on-line control (Scott, 2004). While the short-latency spinal reflex parallels joint velocities, it has been shown that the long-latency response (˜80 ms) involves limb mechanics (Soechting and Lacquiniti, 1988) and is adaptable so as to incorporate the influence of mechanical loads (Burdet et al., 2001; Wang et al., 2001). Also, sensory information is used to direct context-dependent motor responses. For example, it has been shown that small perturbations that push the limb may elicit rapid push or pull motor responses depending on the cued behaviour (Evarts and Tanji, 1976). Thus sensory information is important for a broad range of motor actions.
Another important role for sensory information for motor control is for motor adaptation. For example, during repeated trials of a task an unexpected mechanical load may initially alter limb trajectory. However, subjects are able to modify their motor patterns after a few trials with the load, so that the original limb trajectory is substantially recovered (Lackner and DiZio, 1994; Shadmehr and Mussa-Ivaldi, 1994). If the load is abruptly removed, there is again a deviation in the limb trajectory, and the deviation is a mirror reflection of the perturbation observed when the load was introduced. Recent research suggests that this adaptive process for updating motor patterns for a given movement is strongly dependent on errors in motor performance from the preceding trial (Schiedt et al., 2001), illustrating how sensory feedback from a given movement influences the very next movement.
Clinical assessment of sensorimotor and cognitive function plays a crucial role in all aspects of treating patients, from diagnosing a specific disease or injury, to managing and monitoring rehabilitation strategies to ameliorate dysfunction (Van Dursen and Brent, 1997). The most common clinical assessment technique for proprioception is the Nottingham Sensory Assessment—Revised (Lincoln, 1998). In this technique the clinician positions a joint of the subject's affected limb and asks the subject to mirror the position with the unaffected limb, and then scores the subject's performance (score 0 to 3). In the thumb localizing test (TLT), which is a general proprioceptive test, an examiner holds the affected hand of a subject at a position, and has the subject grasp the thumb of the affected hand with the unaffected hand, with his/her eyes closed (Hirayama et al., 1999; Rand et al., 2001). This is repeated with the affected hand held at different positions, and the subject's performance scored according to a scale (score 0 to 3).
The major challenge with these proprioceptive tests is that they are inherently subjective and have limited resolution. A recent study concluded that the Fugle-Meyer Assessment sensation sub-scale (also based on subjective measures) could not be recommended for clinical use because it showed a significant ceiling effect and low validity and responsiveness to clinically meaningful change (Lin et al., 2004). The ceiling effect implies that many patients attain full score without necessarily having intact sensation.
Some quantitative tests of position sense measure the ability of subjects to actively or passively attain some limb joint angle (Alvemalm et al., 1996; Carey et al., 1996; Elfant, 1977; Carey et al., 2002). For example, the Wrist Position Sense Test (WPST) provides a quantitative measure of wrist position sense in individuals who have had a stroke (Carey et al., 1996; Carey et al., 2002). The WPST is a box-like apparatus with two protractor scales. There is a pointer on top of the box above a protractor (visible to the subject), aligned with the axis of movement of the wrist. There is also an examiner protractor scale, inside the box (hidden from the subject). The subject places an arm in a forearm splint and hand splint which is attached to a lever allowing for movement at the wrist. The examiner imposes wrist movement by moving the lever to different test positions at a relatively constant speed. The subject is unable to see his/her wrist position and the lever. The subject indicates his/her judgment of wrist position by moving the pointer with the other hand or by asking the examiner to move the pointer until he/she believes that it coincides with the wrist angle. The examiner notes the difference between the actual angle (from the hidden protractor) and the perceived angle (from the pointer). Similarly, the work of Brown et al. at the University of Michigan provides apparatus that interfaces with the subject's hand to evaluate proprioception (http://www.kines.umich.edu/research/chmr/mcl.html).
Another example of such a test is the Fully-Automated System by Lonn et al. (1999), which was used to assess position sense at the shoulder in one movement direction. A motorized rig device with a servomotor and a gearbox for different starting and target positions was used. Earphones were given to subjects to receive verbal instructions and minimize auditory cues. The motor rotates the rig to a pre-designated target position and then returns the rig to the starting position. The subject then attempts to replicate the target position and presses a button which registers the matching position. The score is measured as the degree of error between each response and target.
Finally, the Proprioceptometer was designed to quantify position sense changes in the metacarpophalangeal joint (Wycherley et al., 2005). Similar to the WPST, it is a box-like apparatus with a protractor and silhouette (arrow) on top (visible to subject), and an examiner's scale in the middle (hidden from subject). The subject's index finger is isolated in the box out of the subject's view. The subject is asked to match their index finger with the position of the silhouette which is moved in a predetermined sequence. The examiner notes the difference between the perceived and actual angles. In the study by Wycherley et al. (2005), 12 healthy subjects were tested and excellent test-retest reliability was found with this group. However, its validity is unknown. The strengths of this test include the fact that it is a portable device and can be administered in a short time frame (15 minutes). However, a weakness of this test is that individuals with significant deformity of the hand may have difficulty using the apparatus.
A problem with all of the quantitative systems discussed above is that they are limited to motion in a single dimension, and of a single a joint. However, the ability to generate whole-limb motor tasks requires sensory function at multiple joints. Further, impairments may not only reflect sensory impairments at individual joints, but also reflect impairments in the relationship between the limb and its location in space relative to the body (Haggard and Wolpert, 2005).
A number of devices have been proposed for measuring motor performance of the limb. For example, U.S. Pat. No. 6,155,993, issued Dec. 5, 2000 to Scott, relates to a robotic device that can quantify limb movement including motion of the hand and joints and provides joint-based forces to resist limb movement. U.S. Pat. No. 5,210,772, issued Apr. 13, 1993 to Maxwell, relates to a complex linkage which attaches to a subject's limb, and provides forces to resist limb movement. U.S. Pat. No. 5,466,213, issued Nov. 14, 1995 to Hogan et al., relates to a robotic therapist consisting of a computer-controlled mechanical linkage that interfaces with a subject's hand and guides the arm through a range of movement. U.S. Pat. No. 5,830,160, issued Nov. 3, 1998 to Reinkensmeyer relates to a system consisting of a guide that permits limb movement along a linear path. U.S. Pat. No. 6,692,449, issued Feb. 17, 2004 to Brown, relates to a system for assessing limb position of a moving limb. While these systems may be useful in quantifying motor performance or provide motor rehabilitation programs for individuals with impaired movement of limbs, they do not readily provide information relating to sensory impairments of the limb.